The present invention relates generally to the field of implantable medical devices (IMDs), and more particularly to a system and method for determining an estimate of remaining battery life of the IMD.
At present, a wide variety of IMDs are commercially released or proposed for clinical implantation that are programmable in a variety of operating modes and are interrogatable using RF telemetry transmissions. Such medical devices include implantable cardiac pacemakers, cardioverter/defibrillators, cardiomyostimulators, pacemaker/cardioverter/defibrillators, drug delivery systems, cardiac and other physiologic monitors, electrical stimulators including nerve and muscle stimulators, deep brain stimulators, and cochlear implants, and heart assist devices or pumps, etc. Most such IMDs comprise electronic circuitry and an IMD battery that provides power to the electronic circuitry and that depletes in energy over time. Therefore, it is necessary to monitor the state of the battery in such IMDs so that the IMD can be replaced before the battery depletes to a state that renders the IMD inoperable.
Typically, certain therapy delivery and monitoring operational modes and parameters of the IMD are altered temporarily or chronically in a non-invasive (i.e. non-surgical) manner using downlink telemetry transmission from an external programmer of programming and interrogation commands (herein referred to as xe2x80x9cdownlink telemetry dataxe2x80x9d). Moreover, a wide variety of real time and stored physiologic data and non-physiologic data particular to the patient (referred to collectively herein as xe2x80x9cpatient dataxe2x80x9d) can be uplink telemetry transmitted by the IMD to the programmer in response to a downlink telemetry transmitted interrogation command. Other device specific data, including programmed operating modes and parameter values, device state data, and particularly the battery voltage and/or impedance, can also be uplink telemetry transmitted by the IMD to the programmer in response to a downlink telemetry transmitted interrogation command. Such device specific data and patient data are collectively referred to as xe2x80x9cuplink telemetry dataxe2x80x9d.
Since it is often extremely critical for patients"" well-being that IMDs do not cease operating, it is common for IMDs to monitor the level of battery depletion and to provide some indication when the depletion reaches a level at which the battery should be replaced. Pacing implantable pulse generators (IPGs) manufactured by Medtronic, Inc., for example, typically monitor battery energy and depletion and develop an xe2x80x9celective replacement indicatorxe2x80x9d (ERI) when the battery depletion reaches a level such that replacement will soon be needed to avoid further depletion to a battery xe2x80x9cend of lifexe2x80x9d (EOL) condition. Operating circuitry in the pacing IPG typically responds to issuance of an ERI by switching or deactivating operating modes to lower power consumption in order to maximize the ERI-to-EOL interval. For example, internal diagnostic functions and advanced rate-response functions may be discontinued upon issuance of ERI. Additionally, pacing IPG may switch to a relatively low rate, demand pacing mode upon issuance of the ERI as described in commonly assigned U.S. Pat. Nos. 4,390,020, 5,370,668, and 6,016,448, for example. Moreover, the battery impedance, voltage and other indicators of the level of battery depletion can be interrogated during a telemetry session and uplink telemetry transmitted for display and analysis employing the programmer as described above.
In pacing IPGs that monitor battery depletion and provide an ERI, it is important that there be sufficient time between triggering of ERI and complete battery depletion (battery EOL), so that the pacemaker will continue to operate for at least some minimum amount of time after issuance of an ERI. In this way, the physician will have sufficient time to take appropriate action, e.g., to replace the device before battery EOL. At the same time, it is also important not to trigger ERI too early or due to transient faults, since it is desirable that the sudden operational changes associated with ERI not be made until it is actually necessary to do so. Consequently, efforts have been undertaken to avoid issuing an ERI when transient battery states occur that could trigger issuance of the ERI as set forth in the above-referenced ""668 patent, for example, or to derive multi-level ERI indicators as set forth in the above-referenced ""448 patent, for example.
In addition, efforts have been made to derive and provide the physician with a reliable estimate of remaining battery life between the ERI and EOL, sometimes characterized as an elective replacement time (ERT) or a recommended replacement time (RRT) as described in U.S. Pat. No. 5,620,474, for example.
Lithium-Iodine batteries are among the most commonly used power sources in pacing IPGs, and much has come to be known about their depletion characteristics. In particular, it is well known in the art that the output voltage from Lithium-Iodine batteries is relatively flat during early stages of depletion, but drops off rather sharply before EOL. This is due in part to the internal resistance of Lithium-Iodine batteries, which is relatively linear as a function of energy depletion until near EOL, at which time the resistance curve exhibits a xe2x80x9ckneexe2x80x9d where internal resistance begins to rise rapidly. The voltage, capacitance, and impedance characteristics of various Lithium-Iodine cells exhibited over their life times from beginning of life (BOL) are described further in commonly assigned U.S. Pat. No. 5,391,193 and in the above-referenced ""668 and ""448 patents, for example.
The Lithium-iodine battery impedance is history-dependent, i.e. the battery impedance at a point in time following a high rate of discharge of the battery differs from the battery impedance that would be exhibited at the same point in time at a lower rate of discharge. Thus, it is necessary to track the accumulated discharge or current drain that the battery is subjected to from BOL in order to predict the time to ERT or RRT with less uncertainty.
The prior art fairly consistently observes that it is necessary in some way to employ all of these factors in assessing the state of discharge of the Lithium-Iodine battery due to its discharge characteristics However, the use of voltage measurement alone was suggested in U.S. Pat. No. 4,313,079 whereby a battery depletion monitor employs a CMOS inverter to compare the battery voltage to a reference voltage. When the reference voltage exceeds the measured battery voltage, the inverter changes state to indicate battery depletion. However, the loaded terminal voltage of a Lithium-Iodine battery can vary significantly depending upon current consumption due to the internal impedance characteristics discussed above. Thus, if relatively little current is drawn from the Lithium-Iodine battery for a period of time when the battery is nearing but has not reached the ERI point, a sudden prolonged period of high demand on the battery may cause a situation in which too little time is available between the ERI and EOL of the battery. For a particular pacing IPG and lead combination in a given patient, there will be a variation in the effective load on the Lithium-Iodine battery, and a resulting variation in the overall current drain.
Accordingly, if ERI is predicated upon sensing the voltage of the Lithium-Iodine battery and detecting when it drops below a certain level, there can be very little assurance that the level chosen will correspond to the knee of the internal resistance curve. It is therefore necessary to select a high threshold voltage and that unduly shortens the useful life of the pacing IPG.
Many other approaches have been described in the prior art for estimating the remaining life until ERI from measured Lithium-Iodine battery voltage and impedance and also including data related to the operating history of the IMD, e.g., cumulative delivered pacing pulses in the case of pacemakers as described in the above-referenced ""193, and ""448 patents and in U.S. Pat. No. 5,193,538, for example.
More recently developed, implantable grade, Lithium-Carbon Monofluoride or Li(CFx)n batteries have been introduced for use in powering IMDs. High-rate hybrid cathode batteries and cells comprising lithium anodes and cathodes containing mixtures of various types of silver vanadium oxide (SVO) or xe2x80x9ccombination silver vanadium oxidexe2x80x9d (CSVO) and (CFx)n, are disclosed in U.S. Pat. Nos. 5,114,810, 5,180,642, 5,624,767, 5,639,577, 5,667,916, 5,221,453, 5,439,760, 5,306,581 and 5,895,733.
Commonly assigned U.S. Pat. No. 6,157,531 describes embodiments of batteries having lithium anodes, an electrolyte that comprises about 1.0 M LiBF4, and a cathode that comprises about 90% by weight active materials, i.e., 90% by weight of a mixture of (CFx)n and SVO, about 7% by weight polymer binder and about 3% conductive carbon. It is suggested therein that such batteries should be cathode limited to permit accurate, reliable prediction of battery end-of-life on the basis of observing voltage discharge curves since the discharge characteristics of cathode-limited cells are relatively uniform.
However, the determination of the remaining longevity estimate (RLE), that is the time remaining until battery ERI, is not directly possible simply employing direct measurement of battery voltage and extrapolating the remaining battery life from the measured voltage. It is necessary to take into account current consumption factors.
Moreover, it would be desirable to provide an RLE that becomes more and more accurate as battery voltage decreases to assure the physician that the indicated RLE does not overestimate the actual RLE and endanger the patient. The degree of accuracy of the RLE of a fresh and not defective battery at BOL determined at the time of implant would typically be less important than the degree of accuracy of the RLE of a battery approaching EOL.
The present invention utilizes the characteristic features of the battery discharge curves of IMD batteries that are reproducible and predictable under all current drain operating conditions encountered in use of the IMD. These characteristic features include, but are not limited to, a predictable voltage level at a given current drain at all depths of discharge.
The present invention also utilizes an understanding of the drain requirements of the IMD circuitry that are predictable. Third, the cumulative effect of battery, circuit and lead tolerances and errors must be understood and modeled so as to provide valid statistical models for IMD longevity. Under these conditions, the RLE of the IMD battery can be accurately predicted at any point in time from the measured battery voltage and measured or estimated current drain at that time.
In accordance with a preferred embodiment, the IMD measures IMD battery voltage periodically, e.g., every 3 hours, averages the results of every 24 consecutive voltage measurements to produce a running average battery voltage measurement, and maintains the running average battery voltage measurement in IMD memory. The IMD also accumulates battery energy use data, e.g., the count of therapy delivery, e.g., pacing pulses delivered by a pacing IMD, or physiologic monitoring incidents or the like, collectively referred to as an incident count. Each such incident consumes a known battery energy bolus, e.g., the amount of battery energy consumed each time a pacing pulse having known pacing parameters is delivered.
In a telemetry session, a programmer interrogates programmed parameter information and diagnostic data of the IMD including the average battery voltage measurement and current drain indicating data. Current drain indicating data includes background or static or quiescent current drain, e.g., the average energy consumed by the circuitry while monitoring a physiologic parameter of the body, combined with episodic current drain incidents, e.g., delivery of a current consuming therapy through a therapy delivery channel to the body or consumption of energy in monitoring a physiologic parameter. The programmer commands the IMD to initiate measurements of the impedance of the therapy delivery or physiologic monitoring channel. The quiescent current drain can be assumed to be a fixed value determined by the design of the IMD circuitry. The current drain incidents can also be estimated from characteristic operations of the IMD or can be derived from accumulated incident data over a relatively recent time period.
In a pacing system, the delivered pace pulse count of all pacing channels over a predetermined time period, e.g., 72 hours, and the current programmed pacing parameters of each pacing channel are uplink telemetry transmitted to the external programmer from the IMD memory. In the pacing context, the impedance at the output of each pacing pulse generator in each pacing channel is measured by the IMD and uplink telemetry transmitted. Alternatively, the channel impedance may be periodically determined and stored in IMD memory by the IMD itself and uplink telemetry transmitted to the programmer upon receipt of the downlink telemetry transmitted interrogation command.
The programmer computes an xe2x80x9cestimated past current drainxe2x80x9d (EPCD) as a function of the measured channel impedance and the incident data including the energy consumed in each incident and the incident count. The EPCD is the estimated average current drain from the time of the most recent past computation to the present time of computation or a shorter time period. It is not necessary to account for or calculate current accumulated current drain from BOL but only a recent average current drain.
The programmer then computes the RLE (i.e., time xe2x80x9cTxe2x80x9d remaining to ERI) based on the average battery voltage and EPCD. In this case, ERI represents a depleted battery voltage on the characteristic battery discharge curve that precedes further discharge to the EOL voltage that is incapable of adequately powering the IMD circuitry by a predictable number of weeks.
A minimum xe2x80x9ctolerance intervalxe2x80x9d representing a minimum percentile RLE (T_min-past) and a maximum tolerance interval representing a maximum percentile RLE (T_max-past) estimates, as well as the average RLE (T_avg-past), are also computed. The minimum percentile RLE is the T_min-past that represents the RLE during which X % of IMD batteries of the type will discharge to the ERI voltage, where X % can be 5% or any other selected minimum percentage. The maximum percentile RLE is the T_max-past that represents the RLE during which Y % of IMD batteries of the type will discharge to the ERI voltage where Y % can be 95% or any other selected maximum percentage. The computation of T_min-past, T_max-past, and T_avg-past is preferably accomplished using three, fixed 2-dimensional lookup tables, whose indices are battery voltage and EPCD in micro-amps.
The T_min-past, T_-max-past, and the T_avg-past lookup tables preferably represent a plurality of characteristic, predictable battery discharge curves at a respective plurality of EPCD values. The intersection of the EPCD and the battery voltage may fall between plotted look-up table values, and in that case, an interpolation algorithm is invoked to interpolate the T_min-past RLE, T_max-past RLE, and the T_avg-past RLE.
If the IMD is reprogrammed during the current follow-up to adjust pacing energy of future pacing until the next follow-up, then an xe2x80x9cestimated future current drainxe2x80x9d (EFCD), that is likely to be different from the EPCD, is computed. The RLEs are scaled according to the ratio of EPCD to EFCD before being displayed to the user:
Txe2x80x94min-future=Txe2x80x94min-past*(EPCD/EFCD) 
Txe2x80x94max-future=Txe2x80x94max-past*(EPCD/EFCD) 
Txe2x80x94avg-future=Txe2x80x94avg-past*(EPCD/EFCD) 
Note that this scaling method depends on two assumptions about the battery, which are 1) independence of capacity at ERI with respect to current drain, and 2) negligible self-discharge. The scaling equation can be enhanced to accommodate batteries for which these assumptions do not hold.
Thus, preferably maximum, minimum and average RLEs of an IMD battery are determined as a function of battery voltage and EPCD, and the EFCD, if different from the EPCD. The maximum, minimum and average RLEs become more accurate as they shorten as battery voltage decreases, increasing the physician""s confidence in the accuracy of the RLE.
While the maximum, minimum and average RLEs could all be determined within the IMD and uplink telemetry transmitted upon receipt of an interrogation command if sufficient computing power and memory were provided, it is preferable at the present time to share the computation burden and allocate resources between the IMD and an external programmer typically operated by the physician to initiate determination of the maximum, minimum and average RLEs periodically.
This summary of the invention has been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted.